Recent efforts have been made in the field of gasoline blending to increase gasoline octane performance without the addition of deleterious components such as tetraethyl lead and benzene. It has been found that lower molecular weight unsymmetrical ethers such as MTBE and TAME can be added to C.sub.5 -C.sub.10 hydrocarbon-containing gasoline products in order to increase octane number. The research octane number (RON) of MTBE has been listed at 115 (Lander, E. P. et al, "National Petroleum Refiners Association Annual Meeting", San Francisco, Calif., Mar. 20-24, 1983). The blending octane number of MTBE has been calculated over various concentrations and some of the readings are: RON, 115-135; MON (motor octane number), 98-110; and (RON & MON)/2, 106-122.5 (Pecci, G. et al, Hydrocarbon Processing, 1977, 56, 98). Blending octane number rises when MTBE concentration is decreased and saturates concentration of the base fuel is increased.
Conventional etherification processing uses as catalyst a macroreticular cation exchange resin in the hydrogen form. An example of such a catalyst is "Amberlyst 15". A resin catalyst gives a high conversion rate but is unstable at elevated temperatures (above about 90.degree. C.). When overheated, the resin catalyst releases sulfonic and sulfuric acids. In addition leaching of acid substances from the resin catalyst even at normal operating temperatures causes a reverse reaction--decomposition of ether products to starting materials--to occur upon distillation of ether product. Overall yield is thereby significantly decreased (see Takesono et al U.S. Pat. No. 4,182,913).
Etherification reactions conducted over a resin catalyst such as "Amberlyst 15" are usually conducted in the liquid phase below a temperature of about 90.degree. C. and at a pressure of about 200 psig. Equilibrium is more favorable at lower temperatures but the reaction rate decreases significantly. Also excess methanol appears to be required to achieve acceptable selectivity over "Amberlyst 15" (see Chu et al, Industrial Engineering and Chemical Research, vol 26, No. 2, 1987, 365-369).
Some recent efforts in the field of etherification reactions have focused on the use of acid medium-pore zeolite catalyst for highly selective conversion of n-alkene and iso-alkene with lower aliphatic alcohol starting materials. Examples of such zeolite catalysts are ZSM-4, ZSM-5. ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-50 and zeolite Beta. Due to lower acidity as compared to resin catalysts, the zeolites need to be employed at higher reaction temperature to achieve the desired conversion rates. These solid acid catalyst particles are much more thermally stable than resin catalyst, are less sensitive to methanol-to-alkene ratio, give no acid effluent, and are easily and quickly regenerated (see Chu et al, "Preparation of Methyl tert-Butyl Ether (MTBE) over Zeolite Catalysts", Industrial Engineering and Chemical Research, op cit.). Etherification of isoalkenes and n-alkenes with medium pore zeolites is disclosed in U.S. Pat. No. 4,605,787 (Chu and Kuehl) and U.S. Pat. No. 4,714,787 (Bell et al).
Coconversion of etherification effluent from MTBE production is described in U.S. Pat. Nos. 4,788,365; 4,826,507; and 4,854,939 (Harandi et al), wherein unreacted oxygenates and olefins are upgraded in a second zeolite catalyst stage under high temperature reaction conditions to obtain aromatics-rich C5+ hydrocarbon components suitable for use in blending high octane gasoline. This technique is advantageous for eliminating costly methanol and other oxygenates recovery and converting the unconverted butene to higher molecular weight products.
It has been discovered that 2-stage ether and gasoline production can be enhanced by upgrading etherification effluent under controlled moderate severity reaction conditions to oligomerize and etherify isoalkene, thereby producing a second stage effluent rich in high octane C.sub.5 + branched hydrocarbons and additional t-alkyl ether products. Under selected reaction conditions, it is possible to effect the isoalkene oligomerization, etherification and alkylation of olefins with alkanol without forming aromatic or a large amount of low octane linear olefins products. One important aspect of this invention is its ability to remove iso-olefins from linear aliphatics, which typically are blended into gasoline. Iso-olefins are highly reactive gasoline components in the atmosphere, and their removal from gasoline pool hydrocarbons is important in production of clean fuels.